This invention relates to an electroluminescent (EL) panel and, in particular, to an EL panel laminated to another structure having a different coefficient of thermal expansion from the EL panel. As used herein, an EL "panel" is a single strip from which one or more lamps can be made, e.g. by cutting the strip.
An electroluminescent (EL) panel is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include a phosphor powder or there can be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. The front electrode is typically a thin, transparent layer of indium tin oxide or indium oxide and the rear electrode is typically a polymer binder, e.g. polyvinylidene fluoride (PVDF), polyester, vinyl, or epoxy, containing conductive particles such as silver or carbon. The front electrode is applied to a polymer substrate such as a sheet of polyester or polycarbonate to provide mechanical integrity and support for the other layers.
It is often desired to laminate an EL panel with another structure to produce a display. Often, the other structure has a coefficient of thermal expansion different from the polymer substrate, which makes it difficult to maintain critical dimensions along the length of the panel. As an example for the purpose of describing the invention, it is desired to laminate an EL panel having a length of about nine inches and a width of about two inches to an aluminum sheet of about the same size. After lamination, a plurality of lamps are to be punched from the panel and it is desired that the lamps be on precise centers.
The polymer substrate used for EL lamps is typically a bi-axially oriented plastic, meaning that the plastic has been stressed in two, perpendicular directions during manufacture. Uni-axially oriented plastic is used occasionally. In either case, heating the substrate during lamination causes unpredictable changes in dimension as the stress in the plastic is relieved, i.e. the change in dimension is non-linear.
When several lamps are to be made from a panel, electrical connections to the several lamps are provided in the form of patterned electrodes and contact areas. If the panel is laminated, the locations of these areas are no longer precisely known. Existing technology can locate a feature within .+-.0.012" on a panel. This is not sufficient for some applications. Thus, it is difficult to use a laminated EL panel in automatic assembly equipment.
As recognized in the art, automatic equipment requires that cumulative error be small. This requirement is usually met by making each component or performing each step in a process as precisely as possible. High precision parts are more expensive than low precision parts. An alternative is to make equipment adaptive but this is too expensive, at least for existing equipment. Another alternative is to re-design a product to minimize the number of critical dimensions, which is also expensive.
In view of the foregoing, it is therefore an object of the invention to provide a method for laminating EL panels more precisely than in the prior art without increasing the cost of the panel.
Another object of the invention to provide a method for laminating EL panels in which lamps are located within .+-.0.002".